Full Gauge Milling Bottom Hole Assembly with Optimal Contact Force and Build Rate Capability

ABSTRACT

A milling bottom hole assembly (BHA) for use in cutting a full gauge window in a wellbore casing wall, the resultant length of the window being greater than or equal to the whipstock ramp length. A milling BHA is described which includes two shaft portions, a window mill and two bearing mills. The design, which involves strategically placed bearing mills, allows the milling BHA to stay on the whipstock ramp for the entire casing window milling operation and, thereafter, to optimally rapidly build angle and move laterally away from the whipstock and casing, creating a significantly long window which allows for easy passage of directional drilling BHAs through the milled window.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/325,184 filed Nov. 29, 2008 which claims priority toprovisional patent application Ser. No. 60/991,432 filed Nov. 30, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the arrangement and design of millson bottom hole assemblies that are used to cut windows in casing stringsfor the creation of lateral wellbores.

2. Description of the Related Art

In modern hydrocarbon production, it is common to create one or morelateral production wellbores which extend outwardly from a central,generally vertical wellbore. In order to form a lateral productionwellbore, a window must be cut into the side of casing in the centralwellbore. Thereafter, drilling tools are used to form an extendedlateral wellbore. Traditionally, whipstocks and milling tools are usedto create the window in the central wellbore casing wall.

SUMMARY OF THE INVENTION

The invention provides an improved milling bottom hole assembly (BHA)for use in cutting a window in a wellbore casing wall. An exemplarymilling BHA is described which includes a shaft that is made up of twoshaft sections. The distal end of the shaft carries a window mill. Apair of bearing mills is carried by the shaft sections above the windowmill. Preferably, each of the bearing mills is carried by a differentshaft section. Placement of the bearing mills permits the milling BHA tocut a window having a greater length and quality as it allows themilling BHA to stay on the whipstock ramp for the entire millingoperation and then exit the ramp and casing rapidly, such that thelateral build rate of the milling BHA away from the whipstock and itsanchor is optimum and both risks of casing reentry of the milling BHAand excessive damage to the milling BHA are mitigated. The resultantmilled casing exit window is superior for subsequent ingress and egressof long and stiff directional drilling BHAs. A full gaugearrowhead-shaped mill is preferably used for the lower bearing mill. Afull gauge watermelon-shaped mill is preferably used for the upperbearing mill. All three mills, the window mill, the arrowhead-shapedmill and the watermelon-shaped mill, present the same full gaugediameter.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and further aspects of the invention will be readilyappreciated by those of ordinary skill in the art as the same becomesbetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings in whichlike reference characters designate like or similar elements throughoutthe several figures of the drawing and wherein:

FIG. 1 is a side, cross-sectional cutaway drawing of an exemplarymilling BHA constructed in accordance with the present inventiondepicted alongside an associated exemplary whipstock.

FIG. 1A is a side view of an exemplary arrowhead-shaped mill used withthe milling BHA shown in FIG. 1.

FIG. 1B illustrates an exemplary relationship between the angle of thelower portion of the first bearing mill blades and the associatedwhipstock scoop angle.

FIG. 2 is a side, cross-sectional view of an exemplary wellborecontaining the whipstock, and the milling BHA shown in FIG. 1, during aninitial window cutting stage.

FIG. 3 is a side, cross-sectional view of the arrangement depicted inFIG. 2, now with the window cutting operation further advanced.

FIG. 4 is a side, cross-sectional view of the arrangement depicted inFIGS. 2 and 3, now with the window cutting operation further advanced.

FIG. 5 is a graph depicting the correlation of side forces on the windowmill with distance of the window mill from the whipstock kick-off point.

FIG. 6 is a graph depicting an exemplary contact force on a window millas the milling BHA is moved along a whipstock ramp.

FIG. 7 is a graph depicting exemplary contact forces versus distancealong a whipstock ramp.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an exemplary whipstock 10 and a milling BHA 12, whichis constructed in accordance with the present invention. The milling BHA12 includes a threaded upper end 14 which is used for securing themilling BHA 12 to a drill string 16. The milling BHA 12 includes a shaft17 formed of upper and lower shaft sections 18, 20, which are securedtogether at threaded joint 22, and a window mill 24. The window mill 24,of a type known in the art, is secured to the distal end of the millingBHA 12.

A first bearing mill 26 is located on the lower shaft section 20 abovethe window mill 24. The first bearing mill 26 is preferably of fullgauge and is preferably of an arrowhead-shaped configuration, asillustrated in FIG. 1A. The blades of the first bearing mill 26 presentan enlarged, full gauge cutting diameter 25 that is located within theupper half of the length of the mill 26. As a result, the portion 27 aof the first bearing mill 26 that is located above the full gaugediameter 25 quickly increases from the mill's shaft 17 diameter radiallyoutwardly to the full gauge diameter 25. The portion 27 b of the mill 26that is located below the full gauge diameter 25 decreases graduallyfrom the full gauge diameter to the diameter of the shaft 17. Thetapered lower portion 27 b facilitates easy movement and entry of themill 26 onto a whipstock ramp and reduces chances of getting stuck.Also, the positioning of the cutting structures on the gauge section ofthe mill 26 allows effective cutting. The tapered lower portion 27 b isdesigned to improve the longevity of the cutting portion of thearrowhead-shaped first bearing mill 26. In an embodiment, the millingBHA 12 with all milling sections at full gauge diameter is designed suchthat, as the first bearing mill 26 transitions from the primary wellbore44 into the window 40, the contact forces between the first bearing mill26 and the surrounding casing 42 are increased.

Also of note is that the angle of the taper on the lower portion 27 b ofthe arrowhead-shaped first bearing mill 26 is derived from the predictedangular position between the centerlines of the first bearing mill 26and the whipstock 10 when the first bearing mill 26 transitions from theprimary wellbore 44 into the window 40. Because maximum forces areencountered at this transition point, the angle of the taper is suchthat the surface area on the cutting surface is optimized, damage to themill 26's cutting structure is minimized, and cutting structure lifeexpectancy is maximized. FIG. 1B, depicts an exemplary whipstock scoopangle “X,” which is the angle between the vertical axis of the whipstock10 and the inclination of ramp 34 (i.e., the whipstock scoop angle).FIG. 1B also illustrates an angle “Y” which is the angle at which theblades of the lower portion 27 b of the first bearing mill 26 aredisposed from the vertical axis of the milling BHA 12 (i.e., the millblade taper angle). In a currently preferred embodiment, the angle “X”is derived from angle “Y” such that Y=(1.5 to 3)X.

A second bearing mill 28 is located on the upper shaft section 18. Thesecond bearing mill 28 preferably presents a cross-section that iscurved and oblong, thereby presenting a substantially flat centersegment 30 and arcuately curved end sections 32. The second bearing mill28 may be of the type generally known in the industry as a “watermelonmill.” In an alternate embodiment, the second bearing mill 28 presents across-section that is arcuately rounded, in the same manner as the firstbearing mill 26. Both the first and second bearing mills 26, 28 extendradially outwardly to full gauge.

The overall length “L” of the milling BHA 12 (the milling BHA length)exceeds the longitudinal length “l” of the ramp 34 of the whipstock 10(the whipstock ramp length). The second bearing mill 28 is preferablylocated at a distance “x” from the window mill 24 that is from about 1.0to about 1.25 times the length “l” of the ramp 34. Most preferably, thedistance “x” is about 1.15 to about 1.20 times the length “l” of theramp 34. The first bearing mill 26 is preferably located at a distance“d” from the window mill 24 that is from about one-fifth to aboutone-half of the length “x”. Most preferably, the distance “d” is aboutone-third of the length “x”. It is further noted that the spacing (“d1”)between the first and second bearing mills 26, 28 preferably exceeds thedistance “d”.

The distance “x” of the second bearing mill 28 from the window mill 24is also preferably from about 75% to about 90% of the overall millingBHA length “L”. More preferably, the distance “x” is from about 80% toabout 85% of “L”.

FIGS. 2, 3 and 4 illustrate the milling BHA 12 in operation to create awindow 40 in the casing 42 surrounding a primary wellbore 44. FIGS. 2-4also depict the milling BHA 12 exiting the primary wellbore 44 along adeparture path 46 through the surrounding earth 48.

In operation, the drill string 16 and milling BHA 12 are rotated withinthe casing 42, and the milling BHA 12 is lowered within the wellbore 44until the milling BHA 12 encounters the whipstock 10 proximate thekick-off point 43. As FIG. 2 illustrates, the window mill 24 is urgedagainst the casing 42 and begins to cut the window 40. As the millingoperation continues, the window mill 24 cuts downwardly from the upperwindow end 50 to increase the length of the window 40 (as shown in FIGS.3 and 4). At the same time, the incline of ramp 34 urges the window mill24 laterally outside of the wellbore 44. The lower string section 20remains substantially rigid between the window mill 24 and the firstbearing mill 26. However, due to the substantial distance between thefirst and second bearing mills 26, 28, the portion of the lower stringsection 20 above the first bearing mill 26 and the portion of the upperstring section 18 below the second bearing mill 28 will bend and flex.The first bearing mill 26 will cut away the upper end 50 of the window40 during the milling operation, thereby increasing the length of thewindow 40. It is noted that, as the milling operation progresses, thefirst bearing mill 26 will reach the upper end of the whipstock 10before or at the same time as does the mid-point (52 in FIGS. 1 and 3)of the milling BHA 12 due to the spacing of the first bearing mill 26proximate to the window mill 24.

During the milling operation, as illustrated by FIG. 4, the flat portion30 of the second bearing mill 28 will contact the surrounding casing 42and be urged to remain radially inside of the casing 42. This urgingresults in additional lateral forces to be imparted to the lower portionof the milling BHA 12, causing the milling BHA 12 to hold against thewhipstock 10 for a longer time, thus leading to a longer window 40.

The design of the milling BHA 12 provides high constraining forces atthe window mill 24 while it traverses the midsection of the ramp 34 ofthe whipstock 10. The use of a milling BHA 12 constructed in accordancewith the present invention produces a milled window 40 having anextended length, as measured from the upper end 50 to the lower end 52.The proximity of the first bearing mill 26 to the window mill 24 createsrestraining forces on the window mill 24 to urge it properly along thedeparture path 46 from the primary wellbore 44. Additionally, theproximity of the first bearing mill 26 to the window mill 24 helps inharnessing the efficiency of the cutters of the first bearing mill 26for additional cutting of the upper end 50 of the window 40. Thisresults in a longer window 40 than with many conventional techniques.FIG. 3 depicts the upper end 50 of the window 40 being milled away bythe first bearing mill 26. At the same time, the first bearing mill 26is spaced at an optimum distance from the window mill 24 to avoid anearly jump-off of the window mill 24 from the casing 42 near themid-point of the whipstock ramp 34.

As noted, the first bearing mill 26 preferably has an arcuatecross-section, thereby providing for point-type contact between thebearing mill 26 and the surrounding casing 42 or the whipstock 10.Point-type contact results from the fact that the surface of the curvedbearing mill 26 cross-section will contact the surrounding casing 42 orwhipstock 10 at a single point. FIG. 3 illustrates the mill 26contacting the casing 42 at point 54. In addition, the milling BHA 12can pivot with respect to the surrounding casing 42 about the point 54.Binding of the milling BHA 12 as it turns while moving onto the upperend of the whipstock ramp 34 is dramatically reduced as a result of thispoint-type contact between the first bearing mill 26 and the casing 42.The combination of these advantages results in a longer service life forthe milling BHA 12.

FIG. 5 depicts the side forces imparted to the window mill 24 as it ismoved along the whipstock ramp 34 from the kick-off point 43. It can beseen by reference to FIG. 5 that the side forces imparted to the windowmill 24 by the whipstock 10 are kept within a reasonable rangethroughout the milling operation. FIG. 5 is a chart wherein the amountof side force (in kip-force, or klbf) imparted to the window mill (bit)24 is represented by curve 60. As can be seen, the side forces arewithin an acceptable limit and are higher at locations along thewhipstock ramp 34 where the window mill 24 has maximum chances of earlyjump-offs. In FIG. 5, areas where the curve 60 presents a positive sideforce (1, 2, 3, 4, etc.) indicate that the window mill 24 is being urgedagainst the ramp 34 of the whipstock 10. Conversely, areas where thecurve 60 depicts negative side force (−1, −2, −3, etc.) indicate thatthe window mill 24 is being diverted away from the ramp 34 of thewhipstock 10. FIG. 5 indicates that the milling BHA 12 causes the windowmill 24 to be continually urged against the ramp 34 until point 62,which generally coincides with the point at which the window mill 24 hasmoved entirely outside of the casing 42. As a result of this continuouspositive side force, the possibility of the window mill 24 tending toundesirably “jump off” of the ramp 34 during initial phases of windowcutting is minimized. More specifically, when the gauge O.D. of thewindow mill 24 clears the casing 42, because of which the casing 42 nolonger provides a restraining force urging the window mill 24 againstthe ramp 34, side forces are maximized to compensate for the lostcasing-induced restraining force. A thorough finite element analysis ofthe proposed design predicts the trajectory of the lateral bore holecreated in the surrounding earth formation 48 after the window mill 24has moved past the ramp 34 (i.e., beyond point 62 of curve 60). Thisanalysis shows that the window mill 24 and hence the milling BHA 12 willtend to desirably hold or build an angle that is more normal to thecasing 42 than with other milling BHA designs, which tend to drop angle.This improved trajectory is desirable for the subsequent completion of alateral wellbore using a drilling assembly.

It can be seen that the milling BHA 12 and the whipstock 10 collectivelyprovide a window cutting arrangement that is operable to form a windowin surrounding wellbore casing. It should also be understood that theinvention provides an improved method for forming a window withinwellbore casing.

In order to achieve a high build rate, the lower mill 26, which followsthe window mill 24, will experience a contact force/restoring force thatis in a direction towards the whipstock 10 at the time after the windowmill 24 has exited the casing 42. Also, generally the magnitude of thecontact force on the lower mill 26 should be equal to or greater thanthe maximum contact force experienced by the window mill 24. FIG. 6illustrates the contact force upon an exemplary window mill 24 as themilling BHA 12 advances along the ramp 34. The contact force of thewindow mill 24 against the ramp 34 increases gradually (portion 64) asthe window mill 24 enters the whipstock ramp 34. The contact force issubstantially constant during portion 66 as the window mill 24 advancesto the middle of the ramp 34. Finally, as the window mill 24 exits theramp 34, the contact force falls gradually (portion 68).

Contact forces at defined intervals are experienced by the mills 26, 28(which are at drift OD) when they contact the casing 42 as they passthrough the deviated well profile. The contact force plots are generatedfor the window mill 24, lower mill 26 and the upper mill 28. Forcomparison purposes, these respective contact forces are superimposed onthe same plot in FIG. 7. FIG. 7 shows that, when the window mill 24 ison the ramp 34, the contact forces (distance 1-18 in FIG. 7) arepositive, which indicates that the window mill 24 is pressing againstthe whipstock 10. At the same time, the lower mill 26 contact forces arenegative, indicating that it is pressing against the casing 42.Projected distance of the positive window mill contact force curve onthe x-axis is directly proportional to the length of the window thatwill be milled. In the case illustrated by FIG. 7, the positive forcedistance is 19 feet. Once the contact force becomes negative, thisindicates that the window mill 24 has exited the ramp 34 (distance 18-23in FIG. 7). The negative peak on the lower mill 26 contact force(distance 8 in FIG. 7) is seen when the lower mill 26 is just about toenter the whipstock 10. The negative direction also indicates that thelower mill 26 is pressing against the casing 42. It will be appreciatedby one of skill in the art that the window mill 24 experiences acontract force that gradually increases until the window mill 24 reachesapproximately halfway across the whipstock ramp 34 and then graduallydeclines as the first bearing mill passes the upper end of the ramp 34.Once the window mill 24 exits the ramp 34, the lower mill 26 experiencespositive contact forces (distance 21 in FIG. 7), which indicates thatthe lower mill 26 is now pressing against the ramp 34. A highermagnitude of the positive contact force on the lower mill 26 compared tothe negative contact force (distance 21 in FIG. 7) on the window mill 24helps establish the desired build rate for the rat hole that issubsequently drilled.

The foregoing description is directed to particular embodiments of thepresent invention for the purpose of illustration and explanation. Itwill be apparent, however, to one skilled in the art that manymodifications and changes to the embodiment set forth above are possiblewithout departing from the scope and the spirit of the invention.

1. A bottom hole assembly for use in milling a window in a wellborecasing in association with a whipstock with an angled ramp having awhipstock ramp length, the bottom hole assembly having a bottom holeassembly length and comprising: a shaft providing a bottom hole assemblylength; a window mill located proximate a lower end of the shaft; afirst bearing mill upon the shaft; a second bearing mill upon the shaft;the window mill and the second bearing mill being spaced from each otherby a first distance; the first bearing mill being spaced from the windowmill at a second distance that is from about one-fifth to one-half ofthe first distance; and the first distance is from about 75% to about90% of the bottom hole assembly length.
 2. The bottom hole assembly ofclaim 1 wherein the first distance is from about 80% to about 85% of thebottom hole assembly length.
 3. The bottom hole assembly of claim 1wherein the second distance is about one-third of the first distance. 4.The bottom hole assembly of claim 1 wherein the first and second bearingmills are separated from each other by a third distance that is at leastas great as the second distance.
 5. The bottom hole assembly of claim 1wherein the first distance is greater than the whipstock ramp length. 6.The bottom hole assembly of claim 1 wherein the first bearing mill hasan arrowhead-shaped configuration.
 7. The bottom hole assembly of claim6 wherein: the angled ramp presents a whipstock scoop angle; the firstbearing mill includes a lower portion which presents a mill blade taperangle; and the mill blade taper angle is from 1.5 times to 3 times thewhipstock scoop angle.
 8. The bottom hole assembly of claim 1 whereinthe second bearing mill presents a cross-section having a substantiallyflat bearing surface.
 9. The bottom hole assembly of claim 1 wherein thebottom hole assembly length has a midpoint and wherein the first bearingmill reaches an upper end of a whipstock during a milling operationbefore the midpoint of the bottom hole assembly length reaches the upperend of the whipstock.
 10. The bottom hole assembly of claim 1 whereinthe window mill contacts the whipstock to experience a contact forcewhich gradually increases until the window mill reaches halfway acrossthe whipstock ramp and then gradually declines as the first bearing millpasses a top of the ramp.
 11. The bottom hole assembly of claim 1wherein the shaft is formed of first and second shaft sections that arethreaded together and wherein: the first bearing mill is carried by thefirst shaft section; and the second bearing mill is carried by thesecond shaft section.
 12. The bottom hole assembly of claim 1 wherein:the second distance is less than half the length of the ramp; and thefirst distance is greater than half the length of the ramp.
 13. A windowcutting arrangement for forming a window within a wellbore casing, thewindow cutting arrangement comprising: a whipstock to be disposed withinthe wellbore casing, the whipstock presenting an angled ramp and havinga whipstock ramp length; a bottom hole assembly for contacting theangled ramp and cutting a window within the wellbore casing, the bottomhole assembly having a bottom hole assembly length and comprising: ashaft; a window mill proximate a lower end of the shaft and operable forcutting a window in the wellbore casing; a first bearing mill upon theshaft; a second bearing mill upon the shaft; the window mill and thesecond bearing mill being spaced from each other by a first distance;the first bearing mill being spaced from the window mill at a seconddistance that is from about one-fifth to one-half of the first distance;and the first distance is from about 75% to about 90% of the bottom holeassembly length.
 14. The window cutting arrangement of claim 13 whereinthe first distance is from about 80% to about 85% of the bottom holeassembly length.
 15. The window cutting arrangement of claim 13 whereinthe second distance is about one-third of the first distance.
 16. Thewindow cutting arrangement of claim 13 wherein the first and secondbearing mills are separated from each other by a third distance that isat least as great as the second distance.
 17. The window cuttingarrangement of claim 13 wherein the first distance is equal to orgreater than the whipstock ramp length.
 18. The window cuttingarrangement of claim 13 wherein the first bearing mill has anarrowhead-shaped configuration.
 19. The window cutting arrangement ofclaim 13 wherein the second bearing mill presents a cross-section havinga substantially flat bearing surface.
 20. The window cutting arrangementof claim 13 wherein the bottom hole assembly length has a midpoint andwherein the first bearing mill reaches an upper end of the whipstockduring a milling operation before the midpoint of the bottom holeassembly length reaches the upper end of the whipstock.
 21. The windowcutting arrangement of claim 13 wherein the first distance is from about1.0 to about 1.25 times the whipstock ramp length.
 22. The windowcutting arrangement of claim 13 wherein the first distance is from about1.15 to about 1.20 times the whipstock ramp length.
 23. The windowcutting arrangement of claim 13 wherein: the angled ramp presents awhipstock scoop angle; the first bearing mill includes a lower portionwhich presents a mill blade taper angle; and the mill blade taper angleis from 1.5 times to 3 times the whipstock scoop angle.